MAINTAINERS: add OMAP CPUfreq driver to OMAP Power Management section

Add the OMAP CPUFreq driver to the list of files in the OMAP Power
Management section.

I've already been maintaining this driver, this just makes it official.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>